This application claims the priority of German patent 198 40 639.8, filed Sep. 5, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an internal-combustion engine having an engine braking device which comprises brake valves by way of which, in the engine braking phase, the combustion chamber content of the cylinders can be diverted, the brake valves in the engine braking operation being changed into the opening position.
Engine braking systems are preferably used in heavy utility vehicles and consist of a switchable brake valve by way of which, in the engine braking operation, the combustion chamber content can be diverted into the atmosphere or into the exhaust pipe system. In the braking phase, the brake valves are changed into the opening position, the braking performance being generated by the friction-having flow of the combustion chamber gases through the valve opening of the brake valve.
At the high braking performances which can be generated and which, according to the used braking system, may amount to several hundred kW, relatively high rates of air flow and therefore high internal cylinder pressures are generated, which has the result that the brake valves are subjected to high dynamic loads and there is the risk that the operation of the brake valves may be impaired or the brake valves may even become inoperable. However, inoperable brake valves can severely impair the firing operation as well as the engine braking operation.
The invention is based on the problem of increasing the operational reliability of the engine braking device.
According to the invention, this problem is solved by providing a blocking device which acts upon the brake valves and which can be adjusted between a holding position and a release position, wherein in the holding position of the blocking device, the brake valves are held in the open position.
By way of the adjustable blocking device, the brake valves can be held in the opening position in the engine braking operation, whereby brake valve vibrations which may damage the valve are avoided, even at high internal cylinder pressures and correspondingly high flow rates of the combustion chamber content through the valve opening. The brake valves are steadied in their opening position during the engine braking operation and the danger of an impairment of their operation is reduced.
The brake valves are expediently held hydraulically in the opening position, in which case the blocking device can preferably also be operated hydraulically and supplies the brake valves with the necessary hydraulic control pressure. This has the advantage that the control medium of the blocking device is simultaneously the control medium for the brake valves and a no-delay, sensitive adjustment of the control pressure is permitted.
The hydraulic blocking device comprises a pump for conveying the hydraulic medium, a hydraulic supply line for the brake valves as well as an adjustable shut-off valve by way of which the hydraulic supply to the brake valves can be adjusted or blocked. In an advantageous further development, a prestressing device is provided by way of which the hydraulic medium for the brake valves can be acted upon by a definable desired pressure, and a pressure sensor is provided for measuring the hydraulic pressure. By way of the prestressing device, leakage currents can be compensated in the feed pipe supplying the brake valves or in the brake valves themselves. The pressure sensor is required for adjusting the hydraulic pressure to a defined desired value. The prestressing device may be constructed, for example, as a spring-supported piston which acts upon the hydraulic medium. The prestressing device and the pressure sensor may be integrated in a common constructional unit, whereby the construction volume is reduced.
The internal-combustion engine preferably has an exhaust gas turbocharger and a device for the variable adjustment of the exhaust gas back pressure, particularly a variable turbine geometry for the changeable adjustment of the turbine cross-section, in this embodiment, the combustion chamber content being blown off in the engine braking operation into the exhaust pipe system upstream of the turbine. By changing the turbine geometry, it is possible to implement pressures of different intensities in the section between the cylinders and the turbine, whereby the output of the turbine and the performance of the compressor can be adjusted according to the requirements.
In the braking operation of the internal-combustion engine, the turbine cross-section is clearly reduced, whereupon a high excess pressure is built up in the section between the cylinders and the exhaust gas turbocharger. The exhaust gas flows at a high rate through the narrowed-down turbine cross-section and acts upon the turbine wheel, whereupon the combustion air fed to the engine by the compressor is subjected to an excess pressure. An excess pressure also exists on the output side of the cylinder, which excess pressure counteracts the blowing-off of the air compressed in the cylinder by way of the brake valves into the exhaust pipe system. In the engine braking operation, the piston must carry out compression work in the compression stroke and push-out stroke against the high excess pressure in the exhaust pipe system, whereby a strong braking effect is achieved.
By means of the combination of the blocking device with the turbo braking system, it is possible to implement high air flow rates and correspondingly high internal cylinder pressures which are required for achieving high braking performances and to simultaneously prevent a vibration-caused wear of the brake valves.
By way of an automatic control unit, the activities of the turbine, of the brake valves and of the blocking device are preferably coordinated. Expediently, in the event of a braking performance demand by means of the adjusting signals generated by the automatic control unit, the turbine cross-section is reduced by means of the variable turbine geometry, the braking valves are opened simultaneously, the pressure in the blocking device is increased to the desired pressure, and finally the shut-off valve in the hydraulic supply pipe is closed in order to lock the brake valves in the opening position.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.